As a Y pax who's been seduced by WT+ more than once when the numbers look right, I quite like the sound of this idea ...
I can't imagine who BA thinks
dislikes the small cabin. Most J pax seem to clamour to go upstairs, precisely because it is small and intimate - and on the few occasions when I've been lucky enough to be up there, I can see exactly why.
Nevertheless, disembarkation priority shouldn't be a big issue in the decision. If you know what you're doing as a WT+ pax, you can already get off the aircraft ahead of some J pax on many flights. VS' upper deck arrangement presumably also means that their PE pax have to disembark before their UD UC pax - although I've not been there to see it.